The present invention relates to a catalyst for purification of exhaust gases, particularly a catalyst for purification of exhaust gases which is suitably applicable to automobile internal combustion engines.
Various catalysts have been proposed for purification of the nitrogen oxides (NOx), carbon monoxide (CO) and hydrocarbons (HC) present in the exhaust gases emitted from automobile internal combustion engines.
For example, in Japanese Patent Application Kokai (Laid-Open) No. 185450/1988 is disclosed, as a three-way catalyst for use under the severe conditions of high-temperature oxidizing atmosphere, a catalyst for purification of exhaust gases which comprises a honeycomb carrier of one-piece structure and a catalyst composition coated thereon, said catalyst composition containing (a) particles of average particle diameter of 0.5-20 .mu. comprising a refractory inorganic oxide and 5-30% by weight of Pt loaded thereon and/or (b) particles of average particle diameter of 0.5-20 .mu. comprising a refractory inorganic oxide and 1-20% by weight of Rh loaded thereon.
Also in Japanese Patent Application Kokai (Laid-Open) No. 178847/1988 is disclosed a catalyst for purification of exhaust gases which comprises a honeycomb carrier of one-piece structure and a catalyst composition coated thereon, said catalyst composition containing (a) particles of average particle diameter of 0.1-20 .mu. comprising zirconia and 5-30%by weight of Pt and/or Pd loaded thereon and/or (b) particles of average particle diameter of 0.1-20 .mu. comprising zirconia and 1-20% by weight of Rh loaded thereon.
These catalysts are intended to have improved durabilities by loading noble metal(s) on a small amount of a refractory inorganic oxide (e.g. zirconia or active alumina) at high loading ratio(s), making the resulting material into relatively large particles, and dispersing the particles in a catalyst layer made of, for example, active alumina.
Also in Japanese Patent Application Kokai (Laid-Open) No. 295184/1991 by the present applicant is disclosed, as a catalyst to be loaded on a metallic honeycomb structure, a catalyst obtained by loading, on a .gamma.-Al.sub.2 O.sub.3 carrier, 10-100 g/ft.sup.3 of Pt and Pd as catalytic substances.
In Japanese Patent Application Kokai (Laid-Open) No. 224220/1992 also by the present applicant, is disclosed, as a catalyst for use in heater, a mixture of Rh--ZrO.sub.2 and noble metal(s)-heat-resistant inorganic oxide-rare earth element oxide. In the literature, it is described that the composition comprising a heat-resistant inorganic oxide (e.g. alumina) and a rare earth element oxide (e.g. ceria) has 0.1-10% by weight of noble metal(s) (Pt, Pd or a mixture thereof) loaded thereon.
Each of the above-mentioned catalysts, however, has problem(s). The catalyst described in Japanese Patent Application Kokai (Laid-Open) No. 185450/1988 contains no Pd (Pd is important with respect to the catalyst's low-temperature light-off performance particularly for HC) and is therefore unable to effectively convert HC which is emitted in a large amount at the cold start of an engine and which has posed a problem in recent years.
With respect to the catalyst described in Japanese Patent Application Kokai (Laid-Open) No. 178847/1988, its durability is confirmed only in comparison with the catalysts obtained by impregnating a coating layer of active alumina or zirconia with Pt and Rh, and there is seen substantially no effect brought about by loading noble metals on zirconia at a high concentration although there is seen an anti-alloying effect obtained by arranging different noble metals so as to avoid mutual contact. While it is important that the durability improvement of catalyst be made in a state that noble metal(s) is loaded on a carrier in relatively high dispersion, the above literature makes no mention on this point. Further, loading of Pt and/or Pd on zirconia provides no significant improvement in durability and the use of expensive zirconia has no meaning.
The catalyst disclosed in Japanese Patent Application Kokai (Laid-Open) No. 295184/1991 is an ordinary oxidation catalyst and its ability for purification of exhaust gases is not strikingly high.
With respect to the catalyst disclosed in Japanese Patent Application Kokai (Laid-Open) No. 224220/1992, only a wide range is given for the amounts of noble metals loaded and no mention is made on what suitable amounts of Pt, Pd and Rh should be contained in what position of the catalyst layer. Therefore, a satisfactory effect is not consistently obtained always.
The present invention has been completed in view of the above-mentioned problems of the prior art. The object of the present invention is to find out the optimum position(s) of noble metal(s) in catalyst layer, the suitable amount(s) of noble metal(s) loaded, etc. and thereby provide a catalyst for purification of exhaust gases which has excellent purification ability and durability and which enables effective utilization of noble metal(s).